BigIntSum.php

BigInt.js

function LargeInt(asString) {
    this.asString = asString.replace(/^0+/, '');
}

LargeInt.LARGER = 'LARGER';
LargeInt.EQUAL = 'EQUAL';
LargeInt.SMALLER = 'SMALLER';

LargeInt._compare = function(one, another) {
    var digitsOne = one.split(''), 
        digitsAnother = another.split('');

    return  one.asString == another.asString? LargeInt.EQUAL:
            digitsOne.length > digitsAnother.length? LargeInt.LARGER:
            digitsOne.length < digitsAnother.length? LargeInt.SMALLER:
            digitsOne[0] > digitsAnother[0]? LargeInt.LARGER:
            digitsOne[0] < digitsAnother[0]? LargeInt.SMALLER:
            LargeInt._compare(
                one.replace(/^\d/, ''),
                another.replace(/^\d/, '')
            );
}

LargeInt._pad = function(one, another) {
    return  one.length < another.length? this._pad(one.concat('0'), another):
            one.length > another.length? this._pad(one, another.concat('0')):
            [one, another];
}

LargeInt.prototype.add = function(other) {
    var [digitsOne, digitsAnother] = LargeInt._pad(
        this.asString.split('').reverse(), 
        other.asString.split('').reverse()
    );    

    var digits = digitsOne.reduce(function(memo, current, index) {
        var a, b, sum;
        
        a = Number(current);
        b = Number(digitsAnother[index] || 0);
        sum = a + b + memo.next;
        
        return {
            asString: (sum > 9 ? sum - 10 : sum) + memo.asString,
            next: sum > 9 ? 1 : 0
        }       
    }, {
        asString: '',
        next: 0
    }).asString;
    
    return new LargeInt(digits);
}

LargeInt.prototype.compare = function(another) {
    return LargeInt._compare(this.asString, another.asString);
}

LargeInt.prototype.equalThan = function(another) {
    return this.compare(another) == LargeInt.EQUAL;
}

LargeInt.prototype.largerThan = function(another) {
    return this.compare(another) == LargeInt.LARGER;
}

LargeInt.prototype.smallerThan = function(another) {
    return this.compare(another) == LargeInt.SMALLER;
}

LargeInt.prototype.toString = function() {
    return 'LargeInt[' + this.asString + ']';
}

BigIntSum.php

<?php

class BigInt {
    private $number;

    public function __construct($number = '') {
        $this->number = preg_replace('/^0+/', '', $number);
    }

    /**
     * @return BigInt
     */
    public function sum() {
        $memo = $this;

        foreach(func_get_args() as $another) {
            if(is_string($another)) {
                $another = new BigInt($another);
            }

            $memo = self::sumTwo($memo, $another);
        }

        return $memo;
    }

    /**
     * @param BigInt|string $another
     * @return bool
     */
    public function equals($another) {
        if(is_string($another)) {
            $another = new BigInt($another);
        }

        return $this->__toString() === $another->__toString();
    }

    private static function sumTwo($one, $another) {
        list($one, $another) = self::pad($one->number, $another->number);

        $one = str_split(strrev($one));
        $another = str_split(strrev($another));

        $result = '';
        $additional = '';

        foreach($one as $index => $a) {
            $a = (int) $a;
            $b = (int) $another[$index];

            $c = $a + $b + $additional;

            if(strlen($c) == 2) {
                list($additional, $c) = str_split($c);
            } else {
                $additional = '';
            }

            $result .= $c;
        }

        return new self($additional . strrev($result));
    }

    public function __toString() {
        return 'BigInt[' . $this->number . ']';
    }

    private static function pad($one, $another) {
        $oneLength = strlen($one);
        $anotherLength = strlen($another);

        return array(
            $oneLength > $anotherLength ? $one : str_pad($one, $anotherLength, '0', STR_PAD_LEFT),
            $anotherLength > $oneLength ? $another : str_pad($another, $oneLength, '0', STR_PAD_LEFT)
        );
    }
}


$a = new BigInt('1000000000119');
$b = new BigInt('1000000000001');
$c = new BigInt('2000000000120');

$a->sum($b)->equals($c); // true

$small = new BigInt('1');

echo $small->sum('2', '3', '4', '5', '100'); // BigInt[115]